Mode I interfacial toughening through discontinuous interleaves for damage suppression and control

Composites: Part A 43 (2012) 198–207

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Mode I interfacial toughening through discontinuous interleaves for damage suppression and control M. Yasaee a, I.P. Bond a,⇑, R.S. Trask a, E.S. Greenhalgh b a b

Advanced Composites Centre for Innovation and Science (ACCIS), University of Bristol, Queen’s Building, University Walk, Bristol BS8 1TR, UK Composites Centre, Imperial College London, London SW7 2AZ, UK

a r t i c l e

i n f o

Article history: Received 14 April 2011 Received in revised form 30 August 2011 Accepted 6 October 2011 Available online 22 October 2011 Keywords: D. Fractography B. Delamination B. Fracture toughness Crack arrest

a b s t r a c t An investigation is described concerning the interaction of propagating interlaminar cracks with embedded strips of interleaved materials in E-glass fibre reinforced epoxy composites. The approach deploys interlayer strips of a thermoplastic film, thermoplastic particles, chopped fibres, glass/epoxy prepreg, thermoset adhesive film and thermoset adhesive particles ahead of the crack path on mid-plane of Double Cantilever Beam (DCB) specimens. During these mode I tests, the interlayers were observed to confer an apparent increase in the toughness of the host material. The crack arrest performance of individual inclusion types are discussed and the underlying mechanisms for energy absorption and the behaviour of the crack at the interaction point of the interleave edge were analysed using scanning electron microscopy. Ó 2011 Elsevier Ltd. All rights reserved.

1. Introduction Arresting crack propagation in materials is achieved by introducing a feature that is capable of absorbing energy which would have otherwise initiated and propagated cracks [1–3]. To achieve crack arrest during delamination of Fibre Reinforced Polymer (FRP) composites, any feature must be capable of increasing the critical strain energy release rate, GC, of the material. There is limited research in the subject of crack arrest in FRP composite materials. The most common crack arrest features implemented at a structural level are stringers [4], rivets [5], holes [6], and buffer strips [7], which all aid in the redirection and arrest of unstable cracks. Some of these techniques, although successful in homogeneous materials such as steel or aluminium, are not as effective in FRP composites. This is because the additions of structural discontinuities are likely to have a significant detrimental effect on the global mechanical performance of the FRP. This study will focus on the problem and behaviour of delaminations in FRP at a material level, whereby a crack arrest feature is incorporated during the fabrication process using additional materials. Delamination is a common failure mechanism associated with laminated FRPs and is a manifestation of their poor through thickness strength. Comprehensive research has been undertaken into the understanding of delamination in laminated FRPs, with a re⇑ Corresponding author. Tel.: +44 (0) 1173315321. E-mail addresses: [email protected] (M. Yasaee), [email protected] (I.P. Bond), [email protected] (R.S. Trask), [email protected] (E.S. Greenhalgh). 1359-835X/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.compositesa.2011.10.009

cent report by Brunner et al. [8] providing a good indication of overall progress. Delamination can be partitioned into three pure modes of loading; mode I (opening), mode II (shear) and mode III (tearing/twisting) [9], although the latter is usually neglected and is not considered in this study. Combinations of loading modes usually occur to give mixed mode I/II loading. Experimental methods developed to measure the interlaminar fracture toughness of FRP composites under different mode mixities have been standardised for composite materials [10–12]. One method of increasing interlaminar toughness of a composite is by modifying the resin-rich layer in between the plies. This method is called interleaving and has shown to provide considerable improvements in both impact resistance and inter-laminar toughness of an FRP [12–15]. Similarly, crack arrest of delaminations has been achieved by the use of periodically spaced ductile adhesive strips placed in between the plies [7,16–18]. Such strips were successful in arresting delamination; however, testing was only performed under tensile loading. Interleaving will have noticeable effects on other composite properties depending on the extent of implementation within a laminate. Introducing interleaves will result in lower global stiffness, strength and changes to the global fibre volume fraction, Vf, of the composite. Therefore, to mitigate this situation, additional plies must be introduced which in turn will result in a weight penalty [20]. However, within this investigation instead of applying continuous layers of toughening materials, different material types are interleaved as a discrete single strip. To the best of the authors’ knowledge there has been no reported work on the effect of crack propagation from an unmodified interlaminar region of a compos-

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ite into a series of interleaved features. Understanding the mechanisms that cause an apparent increase in the interlaminar toughness of a composite due to the inclusion of discrete pieces of interleaved materials will aid in the development of improved damage tolerant composite materials. This investigation undertakes an experimental assessment of mode I fracture toughness in E-glass reinforced epoxy FRP following the inclusion of various interleaved material inserts. The ability of an individual insert material to increase the strain energy release rate GC, is assessed from the mechanical response and using detailed fractography between the interaction region of the propagating crack and the interleaved feature. 2. Interleaved composites The implementation of interleaved layers does not necessarily add significant cost to the fabrication of a composite, although some interleaves layers, such as those incorporating carbon nanotubes (CNTs), are bespoke and expensive to manufacture. Interleaved materials used for FRP composite fracture toughness enhancements in this study are classified into five different types; additional matrix resin, thermoset adhesive films, random short fibres, micro-sized polymer particulates, and thermoplastic polymer films. The most straightforward method is the use of additional matrix resin on the delamination interface, effectively increasing the interface thickness. This method has been shown to give a substantial improvement in the interlaminar toughness of the composite with GIC and GIIC values increasing by 70% and 200%, respectively, with an interleave thickness of 0.2 mm [15,19]. It was proposed that the GIC increase of such interleaved resin rich interfaces increases with thickness until it reaches a plateau equivalent to the mode I fracture toughness of the matrix material [16]. The plastic yield zone ahead of the crack tip is the essential mechanism that determines the location of the crack propagation through thickness of the interface, as illustrated in Fig. 1. When the interleave thickness is smaller than the diameter of the plastic yield zone the crack will migrate towards the weakest region i.e. the boundary of the fibre resin, resulting in an adhesive failure of the interleave. However, as the interleave thickness increases beyond the plastic yield zone diameter, the crack will no longer propagate along the interface between the interleave and host matrix but will remain within the resin resulting in cohesive failure of the interleave [21] (see Section 5.1). The saturation of the plastic yield zone has been shown to occur for insert thicknesses of >0.1 mm for thermoset resins [22]. Sela et al. [22] looked at the effect on fracture toughness of different thicknesses of interleaved thermosetting adhesive films. They used various film adhesives to show that the inclusion of the interleaves significantly increased both mode I and mode II fracture toughness, a result also confirmed by Gibson et al. [23] and Ozdil and Carlsson [24]. Jiang et al. [25] investigated loading

FRP

Resin interface

Plastic yield zone

(a)

(b)

Fig. 1. Illustration of plastic yield zone interaction with interleaf boundaries for different thicknesses (a) adhesive failure of the resin and fibre and (b) cohesive failure of the resin. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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rate effects on the mode II interleave fracture toughness of Redux 319 adhesive (Hexcel, UK). They found that at lower loading rates, delamination cracking tended to propagate along the boundary between the interleaved layer and the parent composite. However, as loading rate is increased, crack growth remained within the interleaved material which effectively increased toughness. It is clear that the fracture toughness for cohesive failure of the insert or parent resin is generally greater than that of adhesive failure at the fibre/matrix interfaces. Therefore, for thin interface regions, the fracture toughness reflects the bond strength between the matrix and fibres whereas for thicker interface regions, crack jumping is constrained and propagation is in the form of cohesive failure of the resin, hence exhibiting an increase in GIC. The use of additives such as short fibres, nano fibres or polymer particles is another popular method of effectively providing an interleaved layer. Lee et al. [26,27] investigated mode I and mode II fracture toughness improvement using Non-Woven Carbon Tissue (NWCT) as an interleave material. NWCT is ideal for this application due to its good formability and relatively low cost. NWCT contains unaligned short fibres ranging from 3 to 25 mm randomly distributed across the layer. They typically find application in protective layers on exterior surfaces of composite structures. The investigation by Lee et al. [26,27] showed the variation in mode I and mode II fracture toughness (GIC and GIIC) and crack growth location through the thickness of the interleave. Cohesive failure of the interleave led to a significant increase in composite toughness of GIC by 28% and GIIC by up to 260%. They also carried tensile strength analysis of Carbon FRPs (CFRPs) composites interleaved with other types of Non-Woven Tissues (NWTs); these were Non-Woven Polyester Tissue (NWPT), Non-Woven Glass Tissue (NWGT) and Non-Woven Aramid Tissue (NWAT) [28]. Chopped aramid fibres (12–20 mm) were used as an interleave layer with the intention of exploiting the high tensile strain to failure properties of the aramid fibres [29,30]. Improvements in both mode I and mode II fracture toughness were stated as being the result of additional aramid fibre bridging during delamination. Recent advances in the development of nanoscale fibres have led to several investigations for their implementation as interleaved materials. Carbon Nanotubes (CNTs) [31–35], PolyEtherKetone Cardo (PEK-C) nanofibres [36], Nylon-66 nanofibres [37,38], electro spun polysulfone (PSF) nanofibres [39] and b-SiC whiskers [40] have all been evaluated via fracture toughness testing and in all cases significant improvements in GIC and GIIC were reported. The techniques used to manufacture such nanofibre layers were varied and the resulting interleaved layers are always unaligned. However, Wardle et al. [31,32] have developed a novel technique to align CNTs through the thickness and used these as interleaved layers. Subsequent mode I and mode II composite fracture toughness testing exhibited a 300% increase. Spherical nylon particles of varying diameters (10–180 lm) were used as an interlayer material, however, it was reported that the particles capability to sustain thicker interlayer regions helped to increase energy absorption and hence exhibit improvements in mode II fracture toughness [41,42]. In mode I, the fracture toughness was reported to have been reduced as a result of the nylon particle inclusions [43]. Crosslinked carboxyl functionalised elastomer particles based on butadiene acrylonitrile (DuoMod DP 5045, Zeon Chemicals Inc.) are micro particulates which have been developed to improve the fracture toughness of laminates. It was found that interleaving of these particulates in CFRP laminates resulted in GIC and GIIC increases of more than 70% and 350% respectively [44]. Other popular interleave materials include thermoplastic polymer films such as Carboxyl-Terminated Butadiene acryloNitrile (CTBN), PolyUrethane (PU) [45], PolyEthylene Terepthalate (PET) [46–48], Ethylene base ionomer [49,50], Poly Ether Ether Ketone (PEEK) [51] and PolyEtherImide (PEI) [23]. Each material confers

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a relative improvement in fracture toughness on the composite. Some thermoplastic films such as Kapton or PolyTetraFluoroEthylene (PTFE) do not bond to epoxy and are typically used for crack starters in fracture toughness testing. However, Armstrong-Carroll et al. [52] polymerised allylamine monomer in a plasma atmosphere and condensed them onto Kapton and PTFE films. This resulted in a stronger bond to epoxy, subsequent mode II fracture toughness showed a noticeable increase. From the reported studies, interleaved layers offer a very promising technique for increasing interlaminar toughness. Using such layers as strips in the form of a periodic crack arresting feature will be investigated in this study and the propagation of a crack from an unmodified region of the laminate into the interleaved zone will be analysed. 3. Test procedure For this investigation an ASTM-D5528 standard for Double Cantilever Beam (DCB) testing of unidirectional (UD) FRP composites [10] was followed. Due to the low flexural modulus of GFRP there is a possibility of the specimen beam arms undergoing large deflections. Hence, the specimen thickness, h, was chosen to satisfy the following criteria:

 1=3 GIc a20 h P 8:28 E11

ð1Þ

with a maximum expected GIC of 1000 J/m2 at an initial crack length a0 of 50 mm for an modulus E11 of 42.2 GPa. A 24-ply composite laminate of 0° plies of pre-impregnated E-glass fibre/913 epoxy (Hexcel, UK) was selected to give a total thickness of approximately 3.12 mm. A PTFE film of 12 lm thickness was used as a crack starter on the mid-plane of the laminate. An interleaved strip or crack arrest feature of length 10 mm was placed 20 mm ahead of the initial crack front, Fig. 2. Redux 810 epoxy adhesive (Hexcel, UK) was used to adhere two piano hinges to the end of the specimen ready for DCB testing. Using a calibrated Instron test machine with a 18 kN load cell, the load was applied to the specimen at a displacement rate of 4 mm/min. The resulting values of Load (P) and deflection (d) were recorded for every 1 mm increment in crack length a, for the 5 mm prior to interaction with the interleave layer and 5 mm through the interleave strip. Continued crack propagation was measured in 5 mm increments until the crack length reached a total length of 100 mm. The delamination growth was illuminated with a light source placed under the translucent specimen and crack growth was recorded using a digital camcorder placed above the specimen.

This allowed the crack growth to be recorded and any non-linearity in the crack front monitored to distinguish specimens with asymmetric loading. The basic modified version of the beam theory Eq. (2) was used to calculate the GIC of each specimen [10,53].

GIc ¼

3 pd 2 bða þ DÞ

ð2Þ

Five replicates were tested for each fracture toughness assessment of the interleaved layers. 3.1. Interleave materials Initial trials were undertaken using commercially available materials for different interleave types. These trials allowed for optimisation of the most efficient application method of each material type into the GFRP laminates. The thermoset adhesive resin, thermoplastic film and the E-glass/epoxy prepreg strip were cut to size and simply placed on the laminate with the prepreg strip positioned 90° relative to mid-plane fibre direction. The thermoset adhesive resin was found to diffuse along the fibre directions which increased the interleave width from the intended 10 mm to approximately 15 mm. The chopped E-glass and aramid fibres were cut to approximately 10 mm lengths from fibre tows and evenly spread across the laminate with individual fibres randomly oriented. The particulates were weighed and spread onto the laminate to cover the desired area through a fine sieve for even distribution of 143 gsm. Nylon particulates of different average diameters of 20 lm and 30 lm were chosen to distinguish if this had any significant impact on fracture toughness. Chopped glass and aramid fibre allowed a comparison of different fibre strength and strain to failure and the effect of randomness or order. A novel thermoset polymer foam in spherical particulate form of diameters in range of 60–800 lm obtained from High Internal Phase Emulsions (PolyHIPE) [54] was also investigated as such particles could potentially be used to add self-healing function to FRPs. Details of the interleave materials investigated, the thickness of the interleave layers post-cure and the approximate areal density are provided in Table 1. 4. Results The typical DCB load vs. displacement curves of the baseline and the specimens with the interleaved strips are shown in Figs. 3 and 4. Only one of the particulate interleaved samples is plotted.

Fig. 2. DCB test specimen geometry and interleaved strip location.

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M. Yasaee et al. / Composites: Part A 43 (2012) 198–207 Table 1 Interleaved strip material data. Type

Product name

Company

Interleaf cured thickness (mm)

Approximate areal density (gsm)

Thermoset adhesive film Glass fibre Aramid fibre Prepreg strip (base material) Nylon particles (20 lm) Nylon particles (30 lm) Thermoset polymer PolyHIPE particles Polyimide thermoplastic film

Redux 312 Advantex type 30 glass fibre Kevlar 49 E-glass/913 epoxy Polyamide nylon 6 (PA6) Polyamide nylon 12 (PA12) PolyHIPES Surface treated upilex-50RN

Hexcel, UK Owens corning Dupond Hexcel, UK Goodfellow, UK Goodfellow, UK – UBE industries, Ltd.

0.24 0.03–0.7 0.09–0.27 0.133 0.1 0.07 0.125 0.05

159 335 67 160 143 143 143 79

40

30

Load [N]

The point at which the load curve deviates from the baseline sample is indicative of the crack propagation reaching the entry of the interleaved strip. At this point propagation is momentarily arrested until load increase generates enough strain energy to propagate the crack through the interleaved strip. Almost all samples saw an unstable propagation back into the parent material once the crack had reached the end of the interleaved strip. The average critical strain energy release rate, GIC, of each DCB specimen with an interleaved strip was compared with that of the baseline plain GFRP sample at the same crack length, Figs. 5 and 6. The effect of fibre bridging is prominent for mode I delamination in GFRP composites arising from the relatively weak interface between the glass fibres and the epoxy [55]. This effectively increases the apparent fracture toughness of the GFRP samples at longer crack lengths. Compston et al. [56] investigated the influence of fibre volume fraction on the GIC value of GFRP laminates. It was deduced that Vf had little effect on the initiation GIC of the composite, however, it was shown that increasing Vf from 35% towards 55% increased propagation GIC from 400to 780 J/m2 due to the increased amount of fibre bridging. Once the crack had extended to roughly 80 mm, the effect of fibre bridging tended to stabilise and a constant GIC value for the GFRP composite was observed. This value will be referred to as the propagation strain energy release rate, GICProp, which was measured as 684 J/m2. As an illustration of how the fracture toughness of the interleaved region varies, the average critical strain energy release rate plot for a GFRP sample containing a polyimide film is compared with the baseline plain GFRP, Fig. 5. An increase in critical strain energy release rate, GIC, was observed between 70 mm < a < 80 mm. The average GIC rose to a peak at a = 80 mm before unstable delamination

20 Base Polyimide Thermoplastic Film Thermoset Adhesive Film Nylon Particles 20µm

10

0 0

10

20

30

40

50

Displacement [mm] Fig. 4. Load vs. displacement curves for baseline and thermoplastic film, thermoset adhesive, and the 20 lm nylon particulate interleaved strip DCB specimens. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

occurred, after which the GIC returns to the GICProp of the parent material. Similar behaviour was observed for all the interleaved materials that gave an increased strain energy release rate, relative to the baseline GICProp. The peak GIC for all the different interleaved layers occurred at around 80 mm of crack length. The GIC values of the interleaved layers at 80 mm are presented in Fig. 7. The interleaved materials

1500

40

Base

Interleaved Region

Unstable Delamination

Polyimide Thermoplastic Film

1000

GIC [J/m2]

Load [N]

30

20 Base

500

Chopped Aramid Fibres

Chopped Glass Fibres

10

90° Prepreg Strip

0 50

0 0

10

20

30

40

50

Displacement [mm] Fig. 3. Load vs. displacement curves for baseline and chopped aramid, glass and 90° prepreg interleaved strip DCB specimens. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

55

60

65

70

75

80

85

90

95

100

Crack Length [mm] Fig. 5. Mode I Averaged Strain Energy Release Rate (GIC) for baseline plain parent and polyimide thermoplastic film interleaved strip samples (error bars equal one standard deviation, minimum of five samples per data point). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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Au sputter coated and examined using a Hitachi S-3400N VP Scanning Electron Microscope (SEM) at magnifications of 50– 1200 times and voltage accelerations of 15–20 kV, with the surfaces titled by 15° along the longitudinal fibre axis. A schematic of the different areas of interest are shown in Fig. 8. Regions 1 and 5 have similar fracture surface characteristics to the baseline laminate. As the crack moved to region 2, there have been various interactions of the crack with the edge of the interleaved layer and the change in local geometry, hence generating distinct fracture morphologies. Region 3 illustrates how the interleaved material was able to absorb a significant amount of fracture energy whilst the change from the toughened zone to the baseline, region 4, provides some insight into the unstable crack front behaviour as the crack leaves the interleaved region. The fractographic analysis follows established methods of sample preparation and analysis according to the guidelines given in Ref. [57]. The key to the meanings of the annotations used on the SEM images are provided in Fig. 8.

1500 Base

Interleaved Region

Nylon Particles 20µm

GIC [J/m2]

Thermoset foam particles

1000

500

0 50

55

60

65

70

75

80

85

90

95

100

Crack Length [mm] Fig. 6. Mode I Averaged Strain Energy Release Rate (GIC) for baseline plain parent and particulate interleaved strip samples (error bars equal one standard deviation, minimum of five samples per data point). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

5.1. Thermosetting adhesive film The fracture surfaces of the sample with the Redux 312 adhesive interleave strip are shown in Fig. 9. The inclusion of a thermosetting epoxy layer (Redux 312) generated the least disruption to the ply architecture. As the crack moved towards the adhesive rich region, (Fig. 9b), the riverlines that extend through the thickness, changed direction, indicating local mode II loading was present. Also, there was evidence of ductile failure in the adhesive (Fig. 9b). Once in region 3 the uneven pockmarked morphology of the surface of the adhesive, caused by the inclusion of rubber particles, has resulted in highly ductile failure and consequently a more torturous crack path (Fig. 9a). This would be the main contributor to the energy absorption of the resin interleaf. Observation of the fracture surfaces indicates that most energy absorption could be attributed to the ductile nature of the epoxy. The migration of the crack into the interleaved region was smooth with no evidence of inter-ply cracking. The surfaces create similar mode I features as found for the host epoxy resin but with a ductile form of scarps, (Fig. 9c) thus an increase of 43% relative to the GICProp of the baseline laminate was measured.

that offer improved fracture toughness relative to the baseline GICProp are:     

polyimide thermoplastic film (79% increase in GICProp) chopped aramid fibres (46% increase in GICProp) 90° E-glass/epoxy prepreg strip (46% increase in GICProp) thermoset adhesive film (43% increase in GICProp) chopped glass fibres (16% increase in GICProp).

Thermoplastic particles (20 lm and 30 lm Nylon) and thermoset polymer foams (PolyHIPES) reduced the GICProp value by 33%, 27% and 28%, respectively. However, the actual GICProp value was still larger than the initiation GIC of the baseline GFRP laminate, measured as 279 J/m2. This reduction in GICProp is attributed to the inhibition of fibre bridging effect due to the presence of the particles, which is also evident from the slow return to the GICProp value once outside the interleaved region, Fig. 6. Conversely, the greater thickness of the ply interface that these particles confer, possibly contributes to the marginal increase in the initiation value of GIC for the composite.

5.2. Chopped aramid fibre 5. Fractography The fracture surfaces of the sample with the chopped aramid fibre interleave strip are shown in Fig. 10. By observing region 2 of the fracture surface, (Fig. 10c and e), evidence of loose aramid fibres indicates that there has been substantial fibre bridging. Tensile failures of glass fibre and matrix gouges were also visible. As

The most promising interleaved materials were chosen for fractographic analysis. A fully fractured half beam of each DCB specimen was cut using a diamond cutter 10 mm either side of the interleave edge. Both top and bottom fracture surfaces were then

1500

GIC [J/m2]

1200 A B C D E F G H I J

900

600

300

Base (Initiation) Base (Propagation) Thermoset Epoxy Film Chopped Glass Fibres Chopped Aramid Fibres 90° Prepreg Strip Nylon Particles 20µm Nylon Paritcles 30μm Thermoset foam particles Polyimide Thermoplastic Film

0 A

B

C

D

E

F

G

H

I

J

Fig. 7. Apparent average mode I propagation strain energy release rate (GIC) of the crack arrest features (error bars equal one standard deviation, minimum of five samples per data point). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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3. Interleave Region 1. Before Interleave Region 5. Post Interleave Region

bre bridging of the two interfaces contributed to the GIC increase of 46% relative to the GICProp of the baseline laminate. 5.3. Thermoplastic polyimide film

Crack Direction

2. Boundary – Entry to the Interleave Region

4. Boundary – Exit from the Interleave Region SEM Annotation

Meaning Global crack growth direction Local crack growth direction Shear direction

Fig. 8. Schematic illustration of the regions of interest for the delaminated fracture surfaces of the DCB samples.

local cracks around the aramid fibres are formed due to bridging, these tended to migrate towards the global crack front. This phenomenon manifested itself as gouges in the matrix resin. As the crack continued into the interleave layer, region 3 (Fig. 10b), local crack paths could be seen to have differed in direction from the global crack path with many tensile failures of aramid fibres visible. Once the global crack exited the interleave region, local crack jumps, seen as smooth fracture surfaces (Fig. 10a) appear, before converging to typical mode I scarps. From the extensive bridging experienced in the region in the wake of the crack, some degree of interlaminar shear has been introduced to the global mode I failure, this was seen as more pronounced riverlines (Fig. 10a and d). Hence, the increased thickness region and the effective aramid fi-

Uneven surface of the fractured epoxy film

The fracture surfaces of the sample with the thermoplastic polyimide film interleave strip are shown in Fig. 11. In regions 2 and 4 (i.e. before and after the interleaved regions), the fracture surfaces appear similar. A step was generated as the crack travelled from the parent composite through to the interleave region (Fig. 11a, c and e). This was indicative of unstable crack jumps hence the smooth surface with fine riverlines. Immediately under the interleave edge the surface becomes rough. Regions where the polyimide film has been peeled off the matrix resin generates either a smooth surface with shallow wave formation imprinted on the resin (Fig. 11e) or interconnected shallow craters creating a feature consistent with void coalescence (Fig. 11a and b) which would indicate ductility of the film during peel. Evidence of adhesive failure i.e. matrix resin bonded to polyimide film, can be seen in Fig. 11b, which creates a rough surface indicating resin damage, Fig. 11c. As the crack propagates from the parent region into the interleave region, it will propagate either to the top or bottom interface of the polyimide film. This effectively ties the two fracture surfaces with the film material. During loading, energy is required to effectively peel the film from the two surfaces (Fig. 11b). This contributes to a substantial increase in energy absorption as witnessed by the increase of 79% relative to the GICProp of the baseline laminate. Once the crack has propagated fully through the interleave region, evidence of shearing can be seen as shallow cusps and more pronounced step like river lines on the mode I scarps are formed (Fig. 11d).

Ductile fracture of epoxy film

(a) Region 3, 350x 15° Tilt

River line direction

(b) Region 2, 1100x 15° Tilt

Scarps mixture of epoxy film and parent resin

(c) Region 3, 1100x 15° Tilt Fig. 9. Fracture morphology of the mode I DCB sample with thermoset epoxy interleave strip.

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Riverlines

R iv e r lin e s

R e s in r ic h z o n e

Gouges

(c) Region 2, 200x, 15° Tilt

(b) Region 3, 500x, 15° Tilt

(a) Region 4, 250x, 15° Tilt

Loose aramid fibres

Mode I scarps with shear elements

Fibre tensile fracture

(e) Region 2, 1100x, 15° Tilt

(d) Region 5, 500x, 15° Tilt

Fig. 10. Fracture morphology of the mode I DCB sample with chopped aramid fibre interleave strip.

Unpeeled polyimide film

Peeled edge of polyimide film

(a) Region 4, 300x, 15° Tilt

Bonded resin

Shallow Craters (cohesive failure)

(b) Region 3, 300x, 15° Tilt

Resin damage from polyimide film peel

Fibre Sheared

(c) Region 2, 1000x, 15° Tilt

Possible void coalescence

Shallow cusps

Mode I with sheared elements

Peeled polyimide film

Resin rich zone

Riverlines

(d) Region 5, 550x 15° Tilt

(e) Region 2, 300x, 15° Tilt

Fig. 11. Fracture morphology of the mode I DCB sample with polyimide film interleave strip.

M. Yasaee et al. / Composites: Part A 43 (2012) 198–207

5.4. 90° E-glass/epoxy prepreg strip The fracture surfaces of the sample with the 90° E-glass/epoxy prepreg interleaved strip are shown in Fig. 12. The features seen here are identical to the mode I fractured surfaces for a 2D woven fibre composite [57], with evidence of conflicting river lines and scarps indicating many local crack propagation directional changes. As the crack grows through the interleave region, evidence of mode I/II mixity could be seen from the shallow cusps (Fig. 12c) and local shearing of the matrix resin (Fig. 12a and b). River lines can be seen to rotate from a perpendicular direction towards the global crack direction (Fig. 12d). These conflicting river lines and scarps indicate the complex local crack path propagation which aid in arresting propagating global cracks. The effective energy absorption of this sample was similar to that of the aramid samples which is the result of increased interface thickness and fibre bridging. Thus, a GIC increase of 46% relative to the GICProp of the baseline laminates was observed. 6. Discussion It is widely accepted that a composite’s performance can be significantly reduced if minor flaws or defects are present. This sensitivity to defects inhibits the more widespread use of FRPs in safety critical applications. Damage tolerance for composites has been a target for many years, however, in order to realise such a characteristic, the damage needs to be effectively managed. Interleave materials have generally been implemented as toughening mechanisms by introducing them as a continuous layer within a laminated composite. This configuration, although capable of improving delamination resistance of a composite, has a noticeable detrimental effect on other composite properties, such as reduction in global stiffness, fibre volume fraction reduction which leads to subsequent weight penalty as a result [20]. An alternative, more benign approach is to apply interleaved materials as

Mode I scarps with shearing

(a) Region 3, 300x, 15° Tilt

Shallow cusps

(c) Region 4, 1000x, 15° Tilt

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periodic inserts. Using this technique it should be possible to control the distribution and direction of major damage propagation and hence allow an element of damage tolerance to be designed from the outset. For example, during a low velocity impact on a typical FRP, unstable crack propagation occurs from the impact site. This loading condition is typically mode II dominated. If such an impact damaged material is then exposed to axial compression, the delaminations will be seen to propagate in a manner dominated by a mode I loading condition. Therefore, periodic interleaved inserts can be used as means to compartmentalise the structure such that delamination crack growth is arrested at multiple points thereby providing control of subsequent damage. Understanding the precise mechanism by which a propagating crack front interacts with an interleaved insert is an essential requirement to allow the appropriate choice and location of such distributed inserts. In this study, under mode I loading it has been shown that the inclusion of the interleaved inserts markedly increases interlaminar fracture toughness of a GFRP. The effect of these same inserts under mode II loading is reported in an accompanying paper [58] where it is expected shear loading will reduce the dominance of crack bridging effects from the interleave materials; hence the different mechanisms that affect GIIC will be of interest. 7. Conclusion The experimental study described herein aims to determine suitable materials for use as interleaved layers to act as delamination crack arrestors within GFRP composites. The crack arrestors were deployed as 10 mm wide interleaved strips on the mid-plane of 24 ply unidirectional E-glass/epoxy laminates. The different interleave layers investigated included thermoplastic film, thermoplastic particles, chopped E-glass and aramid fibres, E-glass/epoxy pre-preg orthogonally aligned, thermoset adhesive film and

Shearing direction Perpendicular to global crack growth

(b) Region 2, 300x, 15° Tilt

Local perpendicular crack growth direction

(d) Region 3, 300x, 15° Tilt

Fig. 12. Fracture morphology of the mode I DCB sample with 90° prepreg interleaved strip.

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thermoset adhesive particles. From mode I fracture toughness testing, four interleave types were chosen for post-test fractographic analysis due to the significant improvement that was observed in the measured mode I critical strain energy release rate, GIC, relative to a baseline laminate. This investigation indicates that increased interply thickness from the use of an interleaved layer contributes only a small amount to the energy absorption under mode I fracture. However, the maintaining mechanical linkage between the two crack interfaces via the use of inserted fibres or films was seen to be the most effective way of increasing the mode I fracture toughness for a laminated GFRP.

Acknowledgements The authors would like to thank the ESPRC and DSTL for funding of this work under CRASHCOMPS (EP/G003599), Airbus UK for their additional financial support, and Mr. Dan Cegla for supplying the PolyHIPE particulates and for his assistance with the SEM analysis.

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